The subject matter disclosed herein generally relates to optics and, in particular, to multilayer optic devices.
Many imaging applications using photon radiation, such as x-ray radiation, require ever-increasing levels of flux. Increasing x-ray flux may be accomplished, for example, by focusing x-ray radiation emitted by an x-ray source. X-rays can be focused by reflecting an incident x-ray beam 10 from an interface 12 using total internal reflection, as shown in FIG. 1. The interface 12 can be formed between a first material medium 14, typically air, and a second material medium 16, typically a solid. In the illustration, the first material medium 14 has n14 as the real part of its refractive index, and the second material medium 16 has n16 as the real part of its refractive index. Total internal reflection can be realized if n16<n14, and if the angle the incident x-ray 10 makes with the interface 12 is smaller than the critical angle θCR specified for total internal reflection.
The critical angle θCR is determined by the refractive indices of the material media 14 and 16 and the energy of the photons in the incident x-ray beam 10. Generally, the refractive index ‘n’ of matter at x-ray energies can be expressed as n=1−δ+iβ where the term (1−δ) is the real part of the refractive index and the parameter β is related to the absorption coefficient of the corresponding material. At x-ray energies, the real part of the refractive index is very close to unity and is therefore usually expressed in terms of its decrement δ from unity, with δ typically on the order of 10−6 or smaller. At visible wavelengths, the critical angle θCR is largest when the difference between the real part of the refractive indices (n14−n16) or (δ16−δ14) is at a maximum for a given photon energy.
The critical angle θCR for 12.4 keV x-ray radiation incident on an aluminum mirror, for example, is less than 2.7 mrad (approximately 0.15 degree). Thus, to redirect an incident x-ray beam 10 having a beam width ‘w’ of five millimeters, for example, an uncoated planar aluminum mirror having an interface surface 22 length ‘L’ of at least 185 cm would be required, as shown in the illustration. The conventional method of selecting materials solely on the basis of the material indices of refraction produces only modest gains in reflectivity.
The current invention recognizes the need for a reflective multilayer configuration that provides for photon reflectivity at increased critical angles at ultraviolet-ray, x-ray, gamma-ray energies, and for higher gamma-ray energies than has heretofore been realized, and which provides for radiation collection over larger source solid angles than has heretofore been achieved.